Objectives: To provide quantitative models describing the function of pulmonary receptors in their response to P sub CO2, the latter reflecting a gradient of various sensory modalities, from ventilation, through varying ratios of ventilation-to-perfusion, to perfusion-to- metabolic rate. Methods: Microelectrode extracellular recording from soma in the nodose ganglion, the inferior vagal sensory ganglion, of neurons with receptor fields in the lungs, following destruction of all except pulmonary vagal rami. Activity will be calibrated in terms of actual receptor site P sub CO2 following pulmonary artery occlusion. Establishment of standard P sub CO2 static gradient will allow partitioning of P sub CO2 at a sample of receptor sites into (1) "inhaled" P sub CO2, (2) P sub CO2 from respiratory exchange, and (3) P sub CO2 by direct diffusion from the pulmonary artery. The effects of CO sub 2 interchange with respiratory gas-exchange tissue will be estimated, and dynamic responses of individual receptors corrected for waveform distortion of bronchial input P sub CO2 transients. Thus, responses to any input within the physiological range can be described. Finally, correlation of summed individual time-dependent responses to respiratory responses of the intact animal, to intact lungs and arterial blood, and to the vascularly isolated lung will indicate the contribution of the pulmonary receptor system to total animal responses to CO sub 2.